Structures formed of composite materials are widely used in load-bearing structural applications. A composite structure is formed typically by laying a number of thin laminates or plies of fiber-matrix material atop one another to build a part of the desired shape and thickness, and then treating the part to cause the matrix material to bond the various plies together to form an integral structure. Each ply may have only unidirectional fibers, or may have multidirectional fibers. The fibers in different plies may have different orientations.
Composite structures have a number of advantages over non-composite structures. For instance, unlike isotropic materials such as metals, composites can be tailored to have different strength in different directions by varying the number of plies and the angular orientations of the different plies, so that the strength properties of the structure are matched to the load distribution which is expected to be experienced by the structure in use.
Composite materials, however, also have a disadvantage compared to isotropic materials, namely, that the matrix material which holds the fibers together is relatively weak in comparison to the fibers, and therefore composite materials tend to be less damage tolerant than isotropic materials such as metals. Once the matrix material is broken at a given location, such as by impact of the structure by an object, the fibers adjacent that location are no longer held in their desired orientations or "fiber paths." Since the ability of the structure to withstand the expected loads depends on the strength of the fibers and on the fibers being oriented generally along the path of the loads, any deviation of the fibers from their desired orientations can result in substantial failure of the structure.
A particularly weak zone of composite laminates is the interface between two adjacent plies or laminates. There typically is a thin layer of matrix material separating the fibers in one laminate from the fibers in the adjacent laminate. Accordingly, this interface layer or "boundary layer" derives its strength solely from the strength of the matrix material, which, as previously noted, is relatively low compared to the strength of the fibers. For this reason, failure by "delamination," where adjacent laminates separate along their interface, is a particularly troublesome failure mode for composite laminate structures.
Techniques for strengthening composite laminates against delamination have been developed. One technique is to include "third axis" fibers which extend between and through the laminates. For example, a method has been proposed wherein short fibers of carbon or wires of titanium are ultrasonically driven through one face of the laminate structure so that they extend in a thickness direction through the structure. Alternatively, stitching has been used as a means for holding the plies or laminates together and reducing the tendency toward delamination. However, these methods have the disadvantage that they tend to increase the stiffness of the structure, which is undesirable where natural resonant frequencies of the structure must be accurately ascertainable and controlled. Additionally, these methods are relatively cumbersome and expensive.
Moreover, it has been hypothesized, and verified by experimentation, that interlaminar tensile stresses between adjacent laminates are greatest at the free edges of a composite structure and rapidly decrease to near zero at the center. The known techniques for reinforcing composites, such as third axis fibers and stitching, have not taken into account this characteristic of the interlaminar tensile stress distribution.
Additionally, the third axis fiber technique and the stitching technique share the disadvantage that damage is done to the fibers of the composite structure when the third axis fibers or stitches are passed through the structure. Moreover, these reinforcing techniques cannot be used for reinforcing a composite structure formed from pre-cured laminates.